Two demands are-placed on a master cylinder used with a traction control system that brakes driven wheels to control slip. The first demand is that provision must be made to allow the traction control system to draw brake fluid through the master cylinder from a fluid reservoir. The second demand is that provision must be made to allow braking when the system is in an active traction control mode, especially when being used with an antilock braking system (ABS).
Tandem master cylinders are often used in this application to provide adequate hydraulic pressure and to provide braking if a portion of a braking subsystem fails. They must also perform additional functions, which include maintaining zero pressure on each brake when no force is being applied to an associated brake pedal. To meet this objective, compensation ports are used to connect the fluid reservoir with master cylinder pressure chambers. This compensates for the expansion of brake fluid in wheel brake circuits as fluid temperature rises. As the fluid expands, it simply flows into the reservoir to maintain zero pressure on the brakes.
Fluid transferred from the master cylinder pressure chambers to the wheel brake circuits as brake pads and other parts wear must also be replaced. The compensation ports provide the means for meeting this objective. Communication between the fluid reservoir and the master cylinder pressure chambers must, of course, be discontinued during actuation of the master cylinder to allow fluid pressure to be increased. This objective is met by valve action in the master cylinder.
This valve action takes place when the tandem pistons of the master cylinder have been actuated sufficiently to translate respective seals, usually lip seals, mounted thereon past the compensation ports, thereby isolating the pressure chambers from the fluid reservoir. The force that initiates this translation is communicated from the primary to the secondary piston by a pretensioned piston return spring disposed therebetween. Once the seals have been translated past the compensation ports, the force is communicated between the pistons by fluid pressure therebetween. Communication through the compensation ports is restored when force is removed from the brake pedal and the pistons are urged back to their original positions by forces generated by pressurized brake fluid and piston return springs.
To obtain advantages attending being able to draw brake fluid from a master cylinder brake fluid reservoir through the master cylinder for use in a traction control system, previous devices have used master cylinders having relatively complex and expensive center port designs. In addition to being more complex, such designs often use metallic valves and seats, which are more sensitive to contamination than are valves having resilient seals.
Such designs are supplanted by the traction control low-restriction check valve of the present invention, which provides a free path through a compensating port master cylinder without requiring an expensive center port master cylinder.
While the prior devices function with a certain degree of efficiency, none include the advantages attending the use of the traction control low-restriction master cylinder check valve of the present invention, as is hereinafter more fully described.
Further, in prior ABS systems, and particularly those including traction control, the higher fluid pressures created by these systems, where they are fed back into the master cylinder, have made desirable the design feature of minimizing the clearance between the primary and secondary pistons and the housing to preclude undesirable distortion of the elastomeric lip-type fluid seal. Unfortunately, minimizing this clearance undermines being able to "pump up" the brakes as is sometimes desirable to eliminate a soft pedal feel should any appreciable amount of air become entrained in the system. It is this clearance through which the make-up fluid flows from the reservoir to the primary/secondary pressure chamber.